The present invention relates to a method and apparatus for automatically controlling critical variables in welding, especially in automatic or semiautomatic arc welding. The method and apparatus are applicable to various types of welding operations, including the joining edge to edge of flat metal plates, curved plates, beams, bars and the like. A particular application is to the welding of girth joints between hollow annular objects such as lengths of large diameter pipes in the building of pipelines where the work cannot be rotated. It will be understood, of course, that the invention is not limited to this particular application, but the welding of piperline joints is in some respects highly critical and a preferred embodiment of the present invention in the form of a pipeline welding system will therefore be described in detail.
The welding of pipelines is often a highly technical procedure, partly because it must be done in the field under adverse conditions and partly because of the very high standards or specifications required in oil and gas pipelines where leaks or breaks are hazardous and losses of the products are expensive. For many years, such lines have usually been welded manually by highly skilled craftsmen, which is costly and not entirely satisfactory, particularly as specifications and safety requirements have been upgraded. In more recent years automatic or semiautomatic procedures and equipment have come into use; some of these have proved to be quite satisfactory but the best of them encounter problems. Among those which have proved to be the most acceptable are methods and apparatus described in Nelson et al U.S. Pat. Nos. 3,461,264, 3,561,320, and 3,632,959, which involve the making of a first internal weld pass or "stringer bead", the weld later being finished by external passes. For the latter, devices and methods such as described in U.S. Pat. Nos. 3,604,612, 3,718,798 and 3,806,694, for example, have been widely and successfully used. Other procedures have been proposed, as in U.S. Pat. Nos. 3,009,048, 3,084,286, 3,135,850, 3,266,700 and others.
One problem which is common to most of the processes and equipment of the prior art is close control of position of the arc at all times during the welding operation. The most successful processes at present involve a very narrow gap which is to be filled with weld metal. The arc may tend to wander from a true path, due to imperfections in the equipment or lack of precise control of the arc travel. Lateral movements may not properly be controlled when the arc is oscillated as is necessary to fill broader parts of the weld. Associated variables include travel speed, wire feed rate, width of the path or scope of lateral oscillation, alignment of the equipment with the end joint, imperfections in the cutting or preparation of the end surfaces to be joined, and the like. All such variables are important and any or all of them can lead to imperfections in welds.
Small deviations from true center line of travel of the arc along the weld path are often particularly troublesome. Highly skilled operators, controlling the equipment manually can often avoid or minimize the difficulty occasioned by such deviations, but such skilled operators, often are not available. For example, a deep narrow gap must be filled between ends of thick wall pipe. The gap may be less than a quarter of an inch wide. Lateral oscillation of the welder head is required to place a proper weld. Very little deviation from the true or ideal path can be tolerated. Defective welds can result where a side of the gap is either too close or too far from the ends of the lateral traverse of the arc. Prior welders have not been successful in giving automatic and precise guidance or control of width of reciprocation path as the arc travels along narrow gaps.
The present invention is directed to means and a method for maintaining automatic control of the path of travel of the welding head.
Some investigators in the prior art have recognized the problem and have made various efforts to solve it. In U.S. Pat. No. 3,084,264, for example, it was proposed to abut the pipe ends closely together and then to cut a kerf or gap of precise dimensions so as to avoid difficulties associated with variations in width of the gap. In another approach both U.S. Pat. Nos. 3,084,264 and 3,748,433, disclose mechanical feelers which ride along the gap to correct the head position mechanically. One difficulty with such control is that the gap is often irregular or contains small obstructions, such as particles of metal spatter from a previous welding pass, and the like. Attempts have been made to use light sensitive devices to sense and follow the gap.
According to the present invention, means are used to detect certain electrical or analogous phenomena associated with the arc itself, to sense objectionable deviations from the desired travel path before it proceeds so far as to cause trouble, and to initiate corrective action. The corrective action may be mechanical, hydraulic, or otherwise. Automatic operation and control is achieved in the preferred embodiment of the present invention.
In accordance with the invention an apparatus is provided for welding the adjoining ends of annular work members, such as lengths of pipe where a gas shielded arc nozzle serves to melt a depositing metal in the gap between the adjoining ends. A drive motor coupled to the nozzle drives said nozzle laterally back and forth across the gap while the current that flows in said arc during each traverse is integrated to generate a physical representation of magnitude of the integral. Means are provided to arrest the motor each time the integrated magnitude reaches a predetermined threshold value, and to reverse the direction of travel of the motor.